Letter to the Editor

An essential role of RNA editing enzyme ADAR1 in mouse skin

Dyschromatosis symmetrica hereditaria (DSH) and reticulate acropigmentation of Kitamura (RAK) are rare, inherited pigmentary diseases. DSH shows a mixture of pigmented and depigmented macules on the extremities. RAK shows reticulated, slightly depressed pigmented macules on the extremities. The causative gene of DSH was clarified as ADAR1 by positional cloning including linkage analysis and haplotype analysis in 2003. Ten years later, the causative gene of RAK was identified as ADAM10 by whole-exome sequencing, in 2013. ADAR1 is an RNA-editing enzyme which catalyzes the deamination of adenosine to inosine (A-to-I) in double-stranded RNA substrates during post-transcription processing. Inosine acts as guanine during translation, resulting in codon alterations or alternative splice sites that lead to functional changes in proteins when they occur in coding regions. In 2012, it was clarified that ADAR1 mutations cause Aicardi-Goutières syndrome 6, which is a severe genetic inflammatory disease that affects the brain and the skin. A zinc metalloprotease, a disintegrin and metalloprotease domain-containing protein 10 (ADAM10), is involved in the ectodomain shedding of various membrane proteins and shows various functions in vivo. ADAM10 is known to be involved in the ectodomain shedding of Notch proteins as substrates in the skin. We speculate that the pathogenesis of RAK and Dowling-Degos disease (DDD, a pigmentary disease similar to RAK) is associated with the Notch signaling pathway. In addition, ADAM10 mutations proved to be associated with late-onset Alzheimer disease. This review comprehensively discusses the updated pathophysiology of those genetic pigmentary disorders.

Adenosine deaminase acting on RNA 1 (ADAR1) is an essential protein for embryonic liver development. ADAR1 loss is embryonically lethal because of severe liver damage. Although ADAR1 is required in adult livers to prevent liver cell death, as demonstrated by liver-specific conditional knockout (Alb-ADAR1^KO) mice, the mechanism remains elusive. We systematically analyzed Alb-ADAR1^KO mice for liver damage. Differentiation genes and inflammatory pathways were examined in hepatic tissues from Alb-ADAR1^KO and littermate controls. Inducible ADAR1 KO mice were used to validate regulatory effects of ADAR1 on inflammatory cytokines. We found that Alb-ADAR1^KO mice showed dramatic growth retardation and high mortality because of severe structural and functional damage to the liver, which showed overwhelming inflammation, cell death, fibrosis, fatty change, and compensatory regeneration. Simultaneously, Alb-ADAR1^KO showed altered expression of key differentiation genes and significantly higher levels of hepatic inflammatory cytokines, especially type I interferons, which was also verified by inducible ADAR1 knockdown in primary hepatocyte cultures. We conclude that ADAR1 is an essential molecule for maintaining adult liver homeostasis and, in turn, morphological and functional integrity. It inhibits the production of type I interferons and other inflammatory cytokines. Our findings may provide novel insight in the pathogenesis of liver diseases caused by excessive inflammatory responses, including autoimmune hepatitis.